CANADIAN researchers have witnessed the emergence of a conscious thought.
Using imaging scanners to measure brain activity, they recorded the moment when
volunteers became conscious of images appearing in front of them鈥攁nd found
that it all depends on how many neurons are firing in the brain.
Ravi Menon and his colleagues at the University of Western Ontario showed
people patterns of black and white stripes. While keeping the width of the
stripes the same, they increased the contrast slightly with each image they
showed. The volunteers were asked to press a button as soon as they could see a
pattern appearing.
At first, the observers spotted no pattern at all. But when the contrast
reached about 2 per cent they started pressing the button to show that they
could see the stripes. The researchers imaged the brains of the volunteers
continuously during the experiment, using a non-invasive method of brain imaging
called fMRI. This visualises the amount of oxygen in the blood, a measure which
correlates with neural activity. There was an increase in brain activity in the
primary visual cortex of all the volunteers, corresponding to each button press,
and the signal got stronger as the pattern became easier to see. 鈥淭he neurons
fire more as the contrast increases,鈥� says Menon.
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But the researchers could spot the signals even before the volunteers started
pressing the button. 鈥淭hey first pushed the button at 2 per cent. But we saw a
detectable response at 1 per cent,鈥� says Menon. He argues that the difference
between an unconscious response to the stripes and a conscious one is all down
to the level of brain activity. 鈥淵ou need a certain number of neurons to fire to
cross the threshold into consciousness,鈥� he says.
Cognitive neuroscientist David Perrett of the University of St Andrews
agrees. 鈥淐onsidering the number of neurons firing is the only way to think about
consciousness,鈥� he says. 鈥淐onsciousness is brain activity.鈥� But
psychologist Max Velmans of Goldsmiths College in London says that is just part
of the story. He argues that other elements, such as which neurons are firing
and at what frequency, influence consciousness. 鈥淚f you go to the cinema,鈥� he
says, 鈥渁ll of your visual neurons are firing. Then you feel something crawling
up your leg. The signals from your leg in terms of energy are far less than the
flooding of your visual system from the film. But it is more important to attend
to your leg.鈥�
Neuroscientist Susan Greenfield from the University of Oxford says the work
supports her idea that consciousness is like a 鈥渄immer switch鈥� that can be
turned on gradually. 鈥淚 am delighted that they are looking at something
quantitative, so that we can look at degrees of consciousness,鈥� she says.
